The present invention relates to a method of processing pictures of medium or half tone to reproduce them in black and white picture elements.
A typical method for the reproduction of medium tone pictures in black and white picture elements is the dither method. In this method, the density level S(i,j) of each picture element (i,j) constituting a medium tone picture is compared with a corresponding threshold component T.sub.D (k,l) of a threshold matrix (dither matrix) T.sub.D which has M.times.N threshold components. If the density level S(i,j) is larger than or equal to the threshold component T.sub.D (k,l), the picture element will have a logical "1" or black density level when reproduced; if not, a logical "0" or white density level.
Conversion of the picture element (k,j) and threshold component (k,l) is generally carried out according to equations: EQU k=Mod(i,M) Eq. (1)
(if k=0, k=M) EQU l=Mod(j,N) Eq. (2) PA1 (if l=0, l=N)
where Mod(x,y) is a function indicating a residual of division of x by y.
The dither method provides output pictures of a significant quality if input pictures are ture medium tone images. However, the repetitive use of the same threshold component at a predetermined period produces moire fringes should input pictures be dot pictures or dither pictures.
An implement heretofore known to eliminate such moire fringes consists in generating a random number for each picture element to designate a specific threshold component of a threshold matrix. Though succeeding in the elimination of moire fringes, such an implement has a drawback that, since a threshold component is designated by generating a random number for each picture element, the distance between picture elements where the same threshold component is designated is irregular resulting in noticeable noise in output images. Another drawback inherent in this known implement is that the generation of a random number for each picture element requires a high speed random number generator, which increases the cost of the entire apparatus. Stated another way, the total processing rate is dependent on the operation rate of the random number generator.